(a) Field of the Invention
This invention relates to endoscopes and more particularly to an illuminating system for endoscopes having an expanded illuminating range.
(b) Description of the Prior Art
In an endoscope, as the angle of view of the observing system becomes larger, an illuminating system which can illuminate a wider range is required. Recently, an endoscope having an observing system having an angle of view larger than 100 degrees has come to be used and, in response to it, the illuminating system which can illuminate a wider range has come to be required. There is known a conventional endoscope illuminating system which can illuminate a wide range and in which, as shown in FIG. 1, a concave lens 2 is arranged in front of a light guide 1. In the illuminating system having this arrangement, in order to make the range to be illuminated wider, the focal length may be made shorter by curving the concave surface of the concave lens 2 with a larger curvature but, if the concave lens 2 is so made, the brightness on the periphery of the visual field will become so lower than the brightness near the center of the visual field as to be impractical. Further, as shown in FIG. 1, the light coming out of the periphery of the light guide 1 will be refracted so greatly as to hit the peripheral surface 2a and chamfered portion 2b of the concave lens 2, will not contribute to the illumination and will be lost. FIG. 2 shows a relation between the angle of view and the intensity of the illuminating light reaching an object surface. The reason why the brightness on the periphery of the visual field is extremely lower than the brightness near the center of the visual field in the fact that, as shown in FIG. 3, when the ray (of the highest intensity among the rays coming out of the light guide 1) parallel with the fibers of the light guide is considered as a principal ray k, the concave lens 2 will have a strong distortion, that is to say, that the rate d.omega./dh of the increase of the angle of refraction .omega. to the increase of the ray height h of the principal ray k will become quickly large with the increase of h. If considered with the Seidel aberration, the distortion will increase in proportion to the third power of h. Further, in order to avoid such loss of light as is described above, it is necessary to make the diameter of the concave lens 2 large enough. However, it will result in making the diameter of the endoscope large and is not desirable. In order to eliminate such defects, the peripheral surface 2a of the concave lens 2 may be made to be a reflecting surface. However, for that purpose, it is necessary to grind the peripheral surface 2a of the concave lens 2 and evaporatively deposit a metal film on it. However, there are defects that it is a trouble to grind the peripheral surface of the lens, it is difficult to evaporatively deposit a metal on the cylindrical surface and the cost becomes high.